Function Of The Airways Gas Exchange

The principal function of the airways is to permit exchange of gases between blood and the atmosphere that surrounds us, specifically the supply of oxygen to the blood and the removal of carbon dioxide from the blood. This is accomplished by: (1) exchanging gases between the external environment and the alveolar space through breathing or ventilation, and (2) exchanging gases between the alveolar space and the blood by diffusional processes. These two processes are optimized by the structural features of the pulmonary tree. Firstly, the caliber of the airways decreases from the trachea to the alveolus, thereby reducing the dead-space volume. For a given tidal volume (volume of air inspired), decreasing the dead-space volume enhances exchange of atmospheric gases with alveolar gases and results in higher levels of oxygen in the alveolar space and enhanced removal of carbon dioxide from the alveolar space. Secondly, the large surface area shared by the alveoli and the pulmonary capillaries and the short transit distance required for the passage of gases between the alveolar space and the blood enhance gas diffusion.

The direction and extent of passage of gases between the blood in the pulmonary capillary and the alveolar space is determined principally by the gas concentration or partial pressure gradient between these two sites. For example, the partial pressure of oxygen in the alveolus (104mmHg) normally exceeds that in the deoxygenated blood of the pulmonary capillaries (40 mmHg), and therefore oxygen tends to diffuse from the alveolus to the blood (Fig. 2). The opposite is true for carbon dioxide, which has a higher partial pressure in the blood of pulmonary capillaries (45 mmHg) than in the alveolus (40 mmHg), resulting in the diffusion of this gas from the blood into the alveolus. Under optimal conditions for gas exchange, all alveoli would be well ventilated and all pulmonary capillaries would be well perfused. However, not all alveoli are ventilated equally; similarly, not all pulmonary capillaries are perfused to the same degree. An alveolus may be well ventilated, but the associated capillaries may be poorly perfused or not perfused, a situation that may occur as a result of thrombosis, embolization, or compression of pulmonary vessels by high alveolar pressures. The volume of air ventilating unperfused alveolar units during each breath is the alveolar dead space. This volume, together with the anatomic dead space, is known as the physiological dead space. An alveolus may be poorly ventilated (as a result of bronchoconstriction, mucus obstruction, or atelectasis [peripheral airway closure]), and the associated capillaries may be well perfused. In this situation, deoxygenated blood coursing through the pulmonary capillaries is not subject to oxygenation and forms part of a physiological shunt that delivers inadequately oxygenated (or deoxygenated) blood to the left heart. Low alveolar or pulmonary arterial oxygen concentrations induce vasoconstriction [45], which diverts blood away from underventilated alveoli. This process could be viewed as an intrinsic one serving to optimize ventilation-perfusion relationships. Mismatching of ventilation and perfusion can result in less efficient gas exchange between the alveolus and blood. Under normal conditions, ventilation-perfusion of the airways is adequate to maintain the important function of gas exchange and is a composite of the previously mentioned situations.

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